Add tasks for all the new languages

Task/Fibonacci-sequence/Sidef/fibonacci-sequence-1.sidef

@@ -0,0 +1,7 @@
+func fib_iter(n) {
+    var fib = [1, 1];
+    (n - fib.len).times {
+        fib = [fib[-1], fib[-2] + fib[-1]]
+    };
+    fib[-1];
+}

Task/Fibonacci-sequence/Sidef/fibonacci-sequence-2.sidef

@@ -0,0 +1,3 @@
+func fib_rec(n) {
+    n < 2 ? n : (__FUNC__(n-1) + __FUNC__(n-2));
+}

Task/Fibonacci-sequence/Sidef/fibonacci-sequence-3.sidef

@@ -0,0 +1,3 @@
+func fib_mem (n) is cached {
+    n < 2 ? n : (__FUNC__(n-1) + __FUNC__(n-2));
+}

Task/Fibonacci-sequence/Sidef/fibonacci-sequence-4.sidef

@@ -0,0 +1,5 @@
+func fib_closed(n) {
+    define S = (1.25.sqrt + 0.5);
+    define T = (-S + 1);
+    (S**n - T**n) / (-T + S) -> roundf(0);
+}